1. Field of the Invention
The present invention relates generally to the field of superconductor materials and methods for their manufacture. More particularly, the invention relates to the use of Liquid Phase Epitaxy (LPE) methods for growing substrate supported single phase, single crystalline films of the superconductor class based on oxides of Bi, Ca, Sr and Cu, known as BCSCO, and in particular the 80K-phase of that material known as 2122.
2. Description of the Related Art
Since the discovery of Bi--Ca--Sr--Cu--O (BCSCO) superconducting systems in 1988 there have been numerous reports on the thin film growth of this material. High quality epitaxial films of this superconducting material are of vital importance for a large number of applications. Various techniques such as electron beam deposition, magnetron sputtering, molecular beam epitaxy, and laser ablation have been widely used for the growth of thin films of BCSCO superconducting material. These methods are extremely costly and they have not been successful in producing single crystalline thin film specimens. In contrast there have been only a few reports on the growth of these films by the LPE method.
The main advantage of the LPE process over other thin film techniques, with the exception of laser ablation, is the superior stoichiometric control of the grown layers in the film. This is not possible with other methods due to the different vapor pressures of the individual elements of BCSCO at fixed substrate temperatures. Moreover, because of the non-uniform stoichiometric control with these techniques, the films have to be annealed subsequent to the growth to achieve the 80 K. or 110 K. phases. The post deposition annealing treatment of the films imposes a severe restriction on the choice of the substrates because of the possibility of diffusion of substrate constituents into the film.
The substrates that have been so far widely used for growing superconducting films are MgO and SrTiO.sub.3. Although these substrates have a very good structural stability at higher temperatures, they suffer from a large lattice constant mismatch with the 2122 phase of BCSCO. This drawback inhibits the formation of high quality epitaxial films. Recently, LaGaO.sub.3 and NdGaO.sub.3 have been used to grow thin films of BCSCO superconducting material using the LPE technique. Both of these substrates have smaller lattice mismatch with Bi.sub.4 CaSr.sub.2 Cu.sub.2 O.sub.8+x than MgO and SrTiO.sub.3. However, LaGaO.sub.3 suffers the disadvantage of structural instability at higher temperatures due to its phase transition at 875.degree. C. from orthorhombic to rhombohedral structure.
The liquid phase epitaxy method has been used to grow thin films of the 2122-BCSCO phase from Bi.sub.4 Ca.sub.3 Sr.sub.3 Cu.sub.4 O.sub.x charge compositions. Addition of PbO in the initial charge composition has also been used to promote the 80K-phase of BCSCO superconducting film. But the thin film growth of single crystalline 80K-phase material from the supercooled solutions of Bi.sub.2 CaSr.sub.2 Cu.sub.2 O.sub.8+x --KCl system using the LPE process has not been described in the prior art.
High quality and large single crystals of the three leading high-T.sub.c materials, namely, 123 YBCO, BCSCO and Tl-based compounds, have not become available to this date either in bulk or even in thin film form in spite of intensive research to achieve this goal throughout the world in various laboratories. There are some inherent problems associated with all these materials which are very difficult to solve by conventional techniques of crystal growth and synthesis. These problems encountered in producing single crystals of Bi-based superconductors can include 1. incongruent nature of the melt; 2. reactivity of the melt with crucible materials (even with platinum); 3. chemical instability; 4. extreme anisotropy owing to the layered habit and 5. the affinity toward intergrowth. Because of these inherent problems associated with the high-T.sub.c materials the growth of large, homogeneous, single phase, single crystal with no intergrowths has not been possible even though high levels of epitaxy have been achieved.